1. Field of the Invention
The invention is directed to an apparatus for holding an abrasive pad used for abrading the surface of a workpiece.
2. Background Art
The final stages of producing a vehicle brake drum are directed to providing a smooth irregular pattern on the inside of the brake drum using a three-step process. In the first step a single point cutting tool for roughing is applied to bore the inner diameter of the work surface as the drum is rotated. In the second step a single point cutting tool for semi-finishing is applied to the roughed work surface. In both steps, as the tool is fed along the work surface, feed lines form in a spiral pattern. These feed lines are objectionable for commercial applications because a brake pad applied to a surface having this pattern will engage the spiral grooves of the feed lines and be forced to move laterally when the brake is applied causing brake slap. This condition is unacceptable because a lateral force on the brake shoe prematurely wears the material on the brake shoe and also a significant lateral force on the brake shoe may damage the associated brake hardware. For these reasons, these spiral grooves must be eliminated.
Therefore, the third step of the process involves using an abrasive paper applied to the drum surface under pressure by a device that looks similar to a brake drum shoe. During this process a kerosene based coolant is run over the abrasive paper and workpiece. The step involves using a specially built machine that is dedicated to this process as well as additional equipment for cleaning and swarf removal.
A disadvantage of this three-step process exists because one machine is used for the first two steps and a second machine is used for the third step. As a result, the brake drums are transferred from one machine to another and are subjected to runout and imbalance conditions which is referred to as transfer error and which occurs when parts are moved from one machine to another and as a result are chucked on different surfaces of each machine. If the roughing/semi-finishing operations and the surface finish operation could be performed on the same machine, then an improvement of overall part quality as well as a reduction in the number of required machine tools, cycle time and cost would be possible.
An additional factor that may affect the quality of the surface finish is the manner in which the abrasive element, such as the abrasive paper, is supported relative to the workpiece. U.S. Pat. No. 2,606,410 entitled "Device for Superfinishing Machined Surfaces" discloses in FIG. 6 a superfinishing abrasive stone 13 mounted with a pinned connection upon a leaf spring 12 which is urged against the inner surface of a rotating cylinder 44. The stone 13 is not restrained from rotating about the pinned connection and therefore there may not be uniform pressure exerted upon the cylinder 44 across the face of the stone 13.
U.S. Pat. No. 2,244,806 entitled "Honing Apparatus" is directed toward a device for honing the tapered surface 2 of a workpiece "a" using a strip 23 of abrasive material supported by a bar 21. The bar 21 is connected to a spring 19 by a pivot connection using pivot pin 22. Furthermore, the bar 21 is urged against the surface 2 by a single spring 24 seated in a recess 25 of bar 21. While this arrangement pivotally urges the strip 23 against the surface 2, just as with the '410 patent, there may not be uniform pressure exerted upon the surface 2 across the face of the strip 23.
U.S. Pat. No. 2,276,611 entitled "Honing Device" is directed toward a device for honing a conical race 7 of a bearing utilizing a honing stone 12. The patent illustrates in FIG. 1 the use of two coil springs to urge the stone 12 against the race 7. The stone 12 is supported in a spindle and spring pressed outwardly by coil springs. However, there appear to be only two springs utilized which may result in non-uniform pressure exerted upon the race 7 by the stone 12. Furthermore, the stone 12, as illustrated in FIG. 1, is axially restrained by the walls of the spindle in which the stone 12 rests. As a result, it may be possible for the stone 12 to bind within the spindle.
A device is needed to support an abrasive element against a workpiece but permit movement so the element may conform to the shape of the workpiece and also be urged against the workpiece with a relatively uniform pressure across the face of the abrasive element.